1. Field of the Invention
The present invention relates to an optical fiber temperature distribution measuring apparatus, a method for measuring optical fiber temperature distribution, and an optical fiber temperature distribution measuring system, that are for optical and remote measurement of a temperature distribution in an optical fiber.
The present application is based on International Patent Application No. PCT/JP2006/318858 the entire contents of which are incorporated herein by reference.
2. Related Art
As a method for measuring optical fiber temperature distribution for optical and remote measurement of a temperature distribution in an optical fiber, a method in which a principle of distance measuring using an OTDR (Optical Time Domain Reflectometry) is combined with a principle of thermometry by detecting a Raman scattering light is known (please see, for example, patent document 1 and non-patent document 1).
FIGS. 14A and 14B are explanatory diagrams showing a principle of measuring a temperature distribution in an optical fiber disclosed in the patent document 1, wherein FIG. 14A is a diagram showing a principle of a temperature distribution measurement, and FIG. 14B is a graph showing a wavelength distribution of a backscattering light. Measurement of the temperature distribution is conducted by a configuration including a beam splitter 3 which changes an optical path of a backscattering light 6 of an input pulse light 4 transmitted through an optical fiber 2 to be measured, which is generated at a scattering point 5, and outputs the backscattering light 6 as an output light 7 to a wavelength demultiplexer (not shown), a signal detecting unit (not shown) which detects a signal output from the wavelength demultiplexer, and a signal processing unit (not shown) or the like.
Next, a method for measuring a temperature distribution in an optical fiber will be explained below.
Firstly, the input pulse light 4 having a wavelength λ0 generated by a light source (not shown) is input to the optical fiber 2 to be measured, then the backscattering light 6 appears at a certain scattering point 5 in the process of propagation, and returns to an input end side. Herein, when a distance from the input end to the certain scattering point 5 is L, a time elapsed from a time point of inputting the input pulse light 4 to a time point of detecting the backscattering light 6 is t, a refractive index of the optical fiber 2 to be measured is n, a light velocity in vacuum is C0, and a light velocity in the optical fiber 2 to be measured is C,C=C0/n  (1), andL=C·t/2  (2).
Therefore, a position at the scattering point 5 is quantitatively-calculated from the equation (2).
On the other hand, a Rayleigh light 20, a Stokes light 21, and an anti-Stokes light 22 are included in the backscattering light 6 as shown in FIG. 14B. When the wavelength of the input pulse light 4 is λ0, a wavelength of the Rayleigh light 20 is λ0 and a wavelength shift amount is Δλ, a wavelength λS of the Stokes light 21 and a wavelength λAS of the anti-Stokes light 22 are expressed as:λS=λ0+Δλ  (3), andλAS=λ0−Δλ  (4)
Further, when a received light intensity of the Stokes light 21 at the wavelength λS generated at a certain scattering point is IS and a received light intensity of the anti-Stokes light 22 at the wavelength λAS generated at the certain scattering point is IAS, a ratio of the received light intensity IAS of the anti-Stokes light 22 to the received light intensity IS of the Stokes light 21 depends on the absolute temperature T at the scattering point 5 in the optical fiber 1 to be measured, and has a relationship expressed as:IASI/S=A·exp(−h·C·Δλ/kB·T)  (5).
Herein, h is Planck's constant (J·S), Δλ is Ranan shift amount (m−1), kB is Boltzmann constant (J/K), T is the absolute temperature (K), and A is a constant determined by a performance of a measurement system. Therefore, a temperature at the scattering point is quantitatively-calculated. In addition, the anti-Stokes 22 light may be used independently as a function of the absolute temperature T at the scattering point 5 in the optical fiber 2 to be measured, and has a relationship expressed as:IAS=B·(1/(exp(h·C·Δλ/kBT)−1))  (6).
Herein, B is a constant determined by the performance of the measurement system. As described above, the temperature at the scattering point 5 can be quantitatively-calculated.
In addition, the Stokes light and the anti-Stokes light generated at a certain scattering point in the optical fiber distant from the measuring apparatus are attenuated by absorption, scattering and the like by the optical fiber in the propagation of the light through the optical fiber. In the conventional art, the calculated temperature is calibrated considering that attenuation amounts of the Stokes light and the anti-Stokes light in the propagation through the optical fiber per unit distance are constant.
As described above, the position and the temperature at the scattering point can be calculated according to the conventional method for measuring a temperature distribution in the optical fiber.
Patent document 1: Japanese Patent No. 3063063
Non-patent document 1: J. P. Dakin, et al: Distributed Optical Fibre Raman Temperature Sensor using a Semiconductor Light Source and Detector “ELECTRONICS LETTERS” Jun. 20, 1985, Vol. 21 No. 13 p. 569-570
However, according to the conventional method for measuring optical fiber temperature distribution, there is a following disadvantage. FIG. 14A shows the principle for measuring the optical fiber in the conventional art, in that when the optical fiber 2 to be measured is in a hydrogen atmosphere (i.e. hydrogen is contained in a ambient atmosphere 30) for an actual measurement, hydrogen molecules are diffused into the optical fiber 2 to be measured. The back scattering light 6 that is occurred at the scattering point 5 and returned to an apparatus side is absorbed by these diffused hydrogen molecules, so that the received light intensities detected at the signal detecting unit is decreased. Herein, even in a case that the scattering point 5 is not located in the hydrogen atmosphere, if the optical fiber to be measured is in the hydrogen atmosphere at a certain part between the scattering point 5 and the apparatus, the decrease in the received light intensities will occur. An amount of the decrease in the received light intensities due to the hydrogen molecular absorption, namely an amount of increase in an optical transmission loss has a wavelength dependency, so that the received light intensities of the Stokes light and the anti-Stokes light corresponding to a measured temperature at the scattering point include different optical transmission losses due to the hydrogen molecules. As a result, correct temperature information cannot be obtained. In addition, as disclosed in a reference document (N. Uchida and N. Uesugi, “Infrared Optical Loss Increase in Silica Fibers due to Hydrogen”, J. Lightwave Technol., Vol LT-4, No. 8, pp. 1132-1138, August 1986), as to the optical transmission loss in the hydrogen atmosphere, there are an absorption loss due to molecular vibration of the hydrogen molecules diffused in the optical fiber (hydrogen molecular absorption), and a loss such as OH absorption loss due to formation of a hydroxyl group that is the optical transmission loss resulted from a chemical reaction between the hydrogen molecule and the optical fiber. In the present invention, unless described particularly, the increase in the optical transmission loss due the hydrogen molecule means the increase in the optical transmission loss due to the hydrogen molecular absorption.
As an example showing an influence of a presence of the hydrogen molecules on the optical transmission, FIGS. 15A and 15B show a relationship between a distance and a received light intensity detected by the measuring apparatus when the hydrogen molecules exist, wherein FIG. 15A is a characteristic graph of the Stokes light, and FIG. 15B is a characteristic graph of the anti-Stokes light. FIG. 16 shows a relationship between the distance and a thermometric value calculated by the conventional art when the hydrogen molecules exist.
In the relationship between the distance and the received light intensity of the optical fiber to be measured shown in FIGS. 15A and 15B, the characteristics of the Stokes light and the anti-Stokes light under the conditions where a hydrogen partial pressure is varied as 0 MPa, 0.04 MPa, 0.07 MPa, and 0.09 MP are respectively shown.
As shown in FIG. 15A, it is understood that the Stokes light has a tendency that the received light intensity is decreased by attenuation increased in accordance with an increase in a length of the optical fiber to be measured (distance), and that the received light intensity decreases remarkably in accordance with the increase in the hydrogen partial pressure. Further, the same tendency is demonstrated as to the anti-Stokes light as shown in FIG. 15B.
As described above, the optical transmission loss is increased by the diffusion of the hydrogen molecules into the optical fiber to be measured, so that an error occurs between a thermometric value and a true value as shown in FIG. 16. This error is significantly increased in accordance with the increase in the distance of the optical fiber to be measured and the increase in the hydrogen partial pressure, respectively.